text:
Why does a decompression occur when a hole appears on the fuselage of an aircraft?
I've never understood the mechanics behind explosive decompression and what is has to do with the difference of pressure.
I know the pressure in the cabin is set to a certain point, but don't understand what it has to do with why a vacuum occurs after a hole appears in the fuselage of an aircraft.
Could someone explain to me the mechanics behind this?

Not a vaccuum, just a very rapid drop to the pressure of the outside atmosphere at that point.  Which in the stratosphere is quite low.  So, the question contains a misconception, but it still can be answered-- I guess this comment could be made into an answer.

Could you edit your question to clarify where you've searched and what you found already, complete with links and references, and context if applicable? In particular, please let us know what you find missing or unclear about the Wikipedia entry on the topic, if one exists. This allows those who might want to answer to do so without needing to redo the work you've already done.

Haven't you ever popped open a can of beer?  That sound is higher pressure in the can escaping to lower pressure outside the can.  It is the same idea with a pressurized airplane fuselage, except on a larger scale.  And without the beer...

@MichaelHall that's an answer right there... now I'm thirsty.... mmmmmm beeeeeeerrrrrr...

Seriously though, what is the specific question, or concept can you not understand?  Because if you know that airplanes are pressurized, and you understand that there is a pressure differential at altitude, then why wouldn't you instinctively deduce that the pressure would try to equalize when the hull is breached?  Difference in pressure is usually part of the answer, not part of the question...

Tempted to give an answer reading simply "Because a hole has appeared on the fuselage of an aircraft."

I am always puzzled by questions where all the information needed to answer the question is contained within the question itself.  And I am downvoting because a full day later the OP still hasn't clarified his/her confusion...

@MichaelHall  I agree.  "What else do you think would happen???"

Deleting the question is fine, but vandalizing it so that nobody can read what the others are replying to, isn't. They took the time to write replies to you; do them the courtesy of not leaving their answers hanging in limbo, responding to what's no longer visible. Edit rolled back.

If you believe deleting is fine, then would you kindly do so?

@Super If you feel it's necessary, you should be able to delete your own question. However, since others have put effort into answering it, it would be considerate to leave it here. Sure, some of the comments weren't the friendliest, however, if you'd followed the advice of CGCampbell and added your background & research and the specifics you didn't understand, then the answers could have been tailored to explain the things you didn't understand, even if they were "obvious" to others.

Once a question has upvoted answers, it can no longer be deleted by the originator, and this question now has 2 answers with up-votes. Thus, now, deleting isn't the option it once was. The question may possibly be closed, but it will remain for others to search/find later.

Mechanics:
If the air pressure inside the aircraft is high enough compared to the pressure outside,
and if a vent between the two is both large enough and created quickly enough,
then the pressures will equalize (the interior pressure will drop)
more quickly than some structure in the aircraft or in the bodies of those onboard
can handle without serious damage.
Then the decompression might as well be called explosive.

Hollywood misconceptions (not a direct answer, so a link suffices instead of a summary):
https://tvtropes.org/pmwiki/pmwiki.php/Main/ExplosiveDecompression

When a plane is cruising at high altitude, the air pressure outside is much lower than it is at sea level- too low to support life. To prevent the passengers from dying, extra compressed air is pumped into the body of the plane. if a hole appears in the body of the plane, that extra air pressure inside goes rushing out the hole at high speed into the lower air pressure outside.

How big a hole, and how sudden?  For an airliner at cruise altitude, its outflow valve(s) already provide a hole of about a square foot, to avoid overpressurization.

@CamilleGoudeseune  The outflow valve is mostly, like 95% or so, closed at cruise.  I've never tried to drive it open at high altitude (I prefer to keep my job), but if you did you'd get a fairly rapid -- although probably not quite "explosive" -- decompression.  Even with the time (many seconds) to drive the outflow valve open, I'd expect that you could dump the cabin pressure in a minute or less.  That's pretty rapid!

Air pressure is the weight of the air in an imaginary column directly above you up to outer space.  Just like the weight of the water above you increases as you scuba dive into the ocean.
Under normal atmospheric conditions, the weight of the air we feel when sitting on a beach at sea level is 14.7 PSI (pounds per square inch).  Which means there is pressure always pushing against your body from the weight of the air around  you.  As you increase your altitude, the weight of the air above you decreases and the pressure you feel decreases.  As a rule of thumb, the air pressure is cut in half every 18,000 FT.  This would mean the air pressure at 18,000 FT is 7.35 PSI.  (The rule of thumb while scuba diving is the water pressure doubles every 33 feet).
As altitude climbs, there may not be enough oxygen for our body to use and we will become hypoxic (hypoxic hypoxia).  Another characteristic of our body is that our blood needs to have a certain amount of air pressure to push the oxygen into the hemoglobin.  If this cannot occur, will will become hypoxic (hypemic hypoxia).  To maintain a healthy air pressure in the airplane, we pressurize the cabin to an altitude that is suitable for both us and the airplane structure.
There is a misconception with the word "vacuum" that many people have.  Vacuum is not the absence of pressure (in this case air pressure) but a lower pressure than the reference pressure.  So, I could be correct to say that I live in a vacuum because the air pressure at 2500 FT MSL is lower than sea level.
The pressure inside the cabin is at a higher pressure than the outside air pressure the airplane is cruising at.  So in essence, the airplane is also flying in a vacuum (as compared to cabin pressure).  When a hole opens up the air will rush out of the cabin causing the air pressure in the cabin to decrease and cabin altitude to rise.  Air always flows from high pressure to low pressure.
So when an explosive decompression occurs, the higher cabin pressure will rush out to the lower outside air pressure and a vacuum (with respect to where the cabin air pressure was originally) will form, all persons not on oxygen will become hypoxic if there is not enough oxygen in the air.  If air pressure drops to a much lower value, all persons not on a pressure breathing oxygen mask will become hypoxic.

Vacuum:  "A space entirely devoid of matter."   You don't live in a vacuum.

Like I said, there is a misconception on the word "vacuum".  There is no true vacuum.  The definition that I gave it allows the OP to understand the context of what people say when they use that word.  From an engineering standpoint, it is also an accurate definition.

And I'm not a medical pro, but I'm pretty sure hemoglobin doesn't rely on air pressure to "push" the O2 molecule into it.  Hypoxia occurs because there are simply less molecules at altitude.

You can look up those terms I provided and see that they are also correct.

Nobody uses the term vacuum to describe the atmosphere at 2500 feet.  Except you of course.

I think you are missing the bigger picture and getting hung up on an example.  It is only an example and not how I typically think about the air around me.  I simply live in an area where the air pressure is about 13.41 PSIA.  Sea level pressure is 14.7 PSIA so I have a differential (or vacuum) of 1.29 PSIA.  I can understand that you don't like that example.  What example would you use to describe this?  https://www.engineeringtoolbox.com/air-altitude-density-volume-d_195.html

Well, I'm surprised nobody else has jumped on this because this is the sort of site where if you used a term like "the vacuum of space" somebody would argue that it isn't a true vacuum.  True enough, but a lot closer than living at 2500'.  So, I looked up a few definitions of vacuum, and "significant" pressure differential is acceptable.  Personally I would reserve vacuum for more extreme cases.  I used to have to go in a hypobaric chamber every 4 years to experience hypoxia.  They would take us up to 25,000 ft.  I would describe this as significant, but would still avoid calling it a vacuum.

Anyway, I won't debate opinions, I just haven't heard anyone as loose in their interpretation and use of the word before now, and I thought I would offer my perspective in case it helps...

...and I'm not sure what you are asking regarding the link to the engineering tool box.  It is just data and graphs showing decreasing pressure and density with altitude.  At some point at the edge of our atmosphere I would consider it a vacuum, but I don't have any real opinion on what the specific criteria should be.


text:
Why are airplanes parked at the gate with max rudder deflection?
Quite often I see aircraft parked at the gate with rudder deflected to the max. I see this either with Boeing and Airbus aircraft so it must have been done on purpose. I know that there are dampers connected to the yaw, or are those not powerful enough to keep the rudder straight e.g. undeflected?

I don't think this was discussed, but also sometimes deflecting the controls in a certain direction with relation to the wind while the aircraft is stationary helps to keep a sudden gust of wind from flipping or turning the aircraft. This is mostly done on GA aircraft as they are lighter in weight.

I don't have any sources on this so I'll leave it as a comment, but I once heard that large aircraft have a rudder outfitted with a spring (to supress flutter).
When there are no aerodynamic forces the spring force is to only force present, deflecting the rudder.

Yaw damper is connected primarily to the lateral accelerometer, so if the ground is sloping even a bit, it should actually deflect the rudder. However that would interfere with control on the runway, so it is never active on the ground anyway.

The control surfaces of airliners are not connected to the pilots' control via cables; they are operated by a hydraulic pump. When the engines are shut down, there is no hydraulic pressure in the pipes, and the control surface is free to move.
The rudder is moved from the center position by wind. If you observe the gates, you should note that all airplanes have their rudder deflected to the same direction. In a perfect no-wind scenario, the rudder would stay neutral.
There are no gust locks which the pilot must arm before leaving the cockpit on Boeing aircraft (and I presume Airbus too). Gust locks are, however, common in general aviation aircraft. They can also be found on some small business jets.
The reason being, control surfaces on small aircraft work "both ways" - if you move the elevator by hand, the yoke in the cockpit will move as well. These are called "reversible flight control systems". Therefore, when the control surfaces are moved by wind, the entire system - yoke, cables, wheels, bearings etc. are all moved back and forth. This brings a significant amount of wear and stress to the system. Strong wind may also cause the surfaces to deflect past their designed maximum, causing damage.
The control surfaces on airliners are much heavier and much stronger. Also, they are not "directly" connected to the cockpit's controls - when there is no hydraulic power, pretty much every component is disconnected. Unless you're facing gale force winds, it is safe to just let them swing free with wind and gravity.

Could you elaborate on why there is this difference between commercial and general aviation?

On some Boeing aircraft such as the 737, the flight controls do in fact deflect when the surfaces deflect with the wind (or jetblast).  While the surfaces are heavy enough that they aren't going to be banging around the way a light aircraft's surfaces would, there IS deflection in the cockpit.  Which is why, at the gate, one electric hydraulic pump is sometimes left on so that the yoke doesn't randomly move fore/aft when you're sitting in (or climbing into) the seat.

On most tri-cycle gear general aviation aircraft the rudder pedals are used for both the nosewheel steering on the ground and yaw control in the air by moving the rudder. Since they are directly connected, when tied down the nosewheel doesn’t move and the rudder stays in place. (There are exceptions, aircraft like the Grumman Tiger and Cirrus SR22 have castering nosewheels that are not controlled by the rudder pedals. Turns are made with differential braking.)

I have no experience on Boeing or Airbus airplanes but the most likely cause would be gust locks installed on the aircraft.

Some aircraft, like a C-172, have a pin that locks the aileron and elevator in place.  Other airplanes have an externally mounted device locking the rudder in place.

http://www.weekendcfii.com/photos/c172preflt/C172_gust_lock.jpg

Other aircraft have a strap that goes around the flight controls and
rudder.  The strap is tightened and the controls are locked in a
fully deflected state.

http://archive.da.aero/images/intelligence/content/learjet/200911-old_gust_lock.jpg

It would be good to elaborate on why the gust lock is needed. I assume to stop controls flapping around and potentially damaging themselves or something else?

Is this because on GA aircraft they are directly connected to the steering column e.g. the wind would move the stick inside the cockpit, possibly harming an occupant?

Its because uncontrolled slamming of the surfaces damages the control surfaces, linkages, hinges, etc. It really isn't going to do much to anything in the cockpit since there are physical stops for the control column. Some wind can be strong enough to snap control cables or stress them beyond design limits.

When the aircraft is parked there's no hydraulic pressure, which means that the rudder will be deflected by external forces, ie wind.
The ailerons and elevators might get deflected by the wind or their own weight as well.

I'm not sure that this is accurate. Many aircraft have gust locks to avoid this because wind movement can cause damage (and has even been attributed to crashes).

@RonBeyer Most larger aircraft don't have gust locks since the control surfaces are not directly linked to the control inputs in the cockpit (heck, they might not be connected at all in aircraft with FBW system)

Damage to things in the cockpit isn't the point, the control surfaces waving around in the wind damages the linkages at the surfaces themselves. You don't want them just waving around. Hydraulically controlled surfaces don't need one, not because they just let them flap around, but because a depressurized hydraulic controller doesn't allow movement.

@RonBeyer The top voted answers disagrees with you, at least with regard to large commercial jets.

@CareyGregory No, it doesn't.

@VladimirF The top voted answer says control surfaces flapping in the breeze on large jets does no harm. You say the opposite. It says a depressurized hydraulic system allows free movement. You say the opposite. How is that not disagreement?

I say what? You were not very specific before.

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How do check valves regulate the air in a multi-engine airplane's vacuum system?

This is one of the examples of a pneumatic power system in a multi-engine airplane. In systems like this, if one dry air pump fails - let's say the right-hand one - then the check valve manifold closes the right-hand part of the system. I can understand this and it makes sense, but I can't understand how the check valve manifold works.
I already went through an aircraft maintenance manual which has kind of this system, but I can't find any clue. My instructor guesses that inside the check valve manifold there's some kind of shuttle valve. But I don't agree with this. Could someone help me out to understand?

It is probably like any other check valve, it has a spring operated "ball" that seals when pressure on one side allows the spring to expand and push the ball against the seals. I don't have a diagram of the valve (hence not an answer), but that seems plausible.

I agree with you, but I want know what exactly happen inside. I know this is too much for pilot though.

Also, it should be noted that vacuum driven gyro's are commonly being replaced with electronic instruments. As an example, a pair of Garmin G5s in place of the attitude indicator and the directional gyro.  That would simplfy the diagram some.

On each side of the check valve manifold there is a check valve that only allows air to flow toward the vacuum pump on that side.  For the left pump, the check valve is the rectangular bit between the far left line going to the vacuum gauge and the adjacent line going to the deflate valve, and for the right pump it is between the far right line and the gyro suction line.
The result is if one vacuum pump no longer is producing suction, the vacuum gauge indication on that side, being "upstream" of the check valve now that only the opposite pump is pulling air, will show no vacuum; meanwhile the operating pump can still pull air through the other three lines and can still operate the deflate valve and vacuum gyros.  Its vacuum regulator will control the vacuum level.  Everything will continue to work normally with only the loss of vacuum indication for the failed pump providing an indication that something is wrong.

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Is a 100 hour inspection required for an aircraft to be used in a checkride?
Is a 100 hour inspection required when renting an aircraft for a practical test with a DPE, to whom you are paying a fee?
Lots of money is being exchanged, yet the DPE told me that it is not "For Hire" because the money is going to him (i.e. he's not paying me, I'm paying him). How can the direction that the funds are traveling in affect the meaning of the term "for hire"?

I changed the wording and tags slightly on the assumption that you're asking about the US (e.g. DPE is the usual term for an examiner, not DFE). When you ask a question about rules or regulations, please always tell us which country you're asking about. If I got it wrong, you can just roll back or edit again.

As far as the FAA is concerned the DPE is neither a passenger nor an instructor. It is as if you are flying solo. The fact that money changes hands has nothing to do with whether a 100 hour inspection is needed. What matters is how the organization that provided the airplane operates.

You're not making money from your aircraft, so it's not a commercial operation. Just like if you paid someone to inspect your house, that doesn't mean you're renting it to them.

Abelenky's answer is right, but the relevant regulation is FAR 91.409(b):

(b) Except as provided in paragraph (c) of this section, no person may operate an aircraft carrying any person (other than a crewmember) for hire, and no person may give flight instruction for hire in an aircraft which that person provides, unless within the preceding 100 hours of time in service the aircraft has received an annual or 100-hour inspection and been approved for return to service in accordance with part 43 of this chapter or has received an inspection for the issuance of an airworthiness certificate in accordance with part 21 of this chapter. The 100-hour limitation may be exceeded by not more than 10 hours while en route to reach a place where the inspection can be done. The excess time used to reach a place where the inspection can be done must be included in computing the next 100 hours of time in service.

So here are the rules:

No person may operate an aircraft carrying any person for hire (for hire here refers to the pilot, not the passenger, and the person operating the aircraft is the pilot)
No person may give flight instruction for hire (here person refers to the flight school or an instructor which the flight instructor or flight school provides) without a 100 hour inspection.

This means you can hire a flight instructor to instruct in your own aircraft without a 100-hour inspection. You can also fly a rented aircraft without a 100 hour so long as that flight is not under the guidance of a flight instructor.
Since you were not being paid, and you were not being instructed, the 100 hour inspection does not apply. However in practice I don't know of any aircraft rental service that allows the planes to fly beyond 100 hours regardless of it being an instructional flight or a private one.

Why do you say you can't receive instruction? "no person may give flight instruction for hire in an aircraft which that person provides" This seems to imply that you can receive instruction.

@Zaz Last I checked, to receive instruction you needed both the student and an instructor in the aircraft (not counting solo's). Since the instructor can't give instruction, you can't receive it either. By the way the person providing the aircraft in what you cite is the instructor, not the student. So you can receive (and give) instruction in an aircraft that the student provides, but not one that the instructor provides unless it has a 100 hour inspection.

What I'm saying is that you can rent a plane, hire an instructor, and then receive instruction in that plane. If the student is renting the plane, then the instructor is not providing it, right?

@Zaz That's true, but you can't rent an aircraft unless it has a 100 hour inspection either... Whether using it for instruction or not, so its a moot point.

Your answer seems to say you can: "You can also fly a rented aircraft without a 100 hour ..." ?

The rule is that the pilot may not be "for hire".
He is for hire, but as a DFE, not the PIC (the one flying the plane), so he is not breaking the rule.
You are the one flying the plane (the PIC), but you aren't getting paid (in fact, you are paying someone else), so you aren't "for hire" either.

So does this mean that if he decides to end the PT in flight, and take control of the aircraft, he cannot, as he would then be assuming PIC role and since he is getting paid, he would be violating the rule? I know this sounds silly, but my DFE made my ignorance of this an issue on a PT I took recently.

If the examiner ends the check ride in flight and takes control, the function that he is being paid for has been terminated.  He is not "flying for hire" at that point; he's just saving his skin as it were.

@abelenky This is not right. The plane can be for hire or the pilot can be for hire. But the person or organization providing the plane cannot also provide the pilot. So I can hire a pilot to fly my plane without needing a 100 hour inspection. I can also rent out my airplane without needing a 100 hour inspection. What I can’t do is provide a plane and pilot for hire—that would be part 135. I also cannot provide a plane and instructor—that would fall under CFR 91.409.

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Inviscid aerodynamics: why is circulation zero at the wingtip?
I read that the circulation at the wingtip for a finite wing is $0$. I am a little bit confused about this statement. First, it is said that drag due to lift (aka induced drag) generates vortices at the wingtip. But on the other hand in Prandtl's lifting line theory the circulation at the wingtip dies off. I thought about some explanations but I would like to know which one is really true. I also welcome any additional explanations.

Pressure difference at wingtip is $0 \implies $ no lift $\implies \Gamma = 0$.
Indirect: If the circulation at the wingtip is not equal to $0$, then the downwash due to wingtip vortex is infinitely large, which is not possible. This gives $\Gamma =0$ by contradiction.

Prandtl's lifting line assumes that the form of the solution for its equation is similar to an elliptical solution with small variations.
$$
\Gamma(\Theta)=4sV_{\infty}
\sum_{n=1}^{\infty}A_nsin(n\Theta)
$$
Which, essentially, assumes that the circulation at the tips is 0, so the tip is not producing lift at all in any assumemd solution. The theory assumes that the tip pressure difference is 0 and actually no speed normal to the surface.
This theory explains that the drag due to lift is NOT a drag produced at the tip, is drag of the complete wing as a single element and the tip vortex is not produced at the vortex, is produce down the wing due to the circulation variation along the wing.
Which drives the typical picture (via helicel)

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When/where must a flight go missed on the JFK VOR/GPS 13 L/R approach?
I'm looking at "exciting" approach plates, and I always come back to JFK VOR/GPS 13L/R.

I think I have come to understand most of the elements of this unusual approach, and enjoy seeing flights make that "dramatic" right turn just before landing, but here is what I can't figure out, even after reading over the plate multiple times:
With such a long distance (relatively) from the MAP to the runway, and additional features such as lead-in lights involved, when and where should the pilot consider, or decide, to break off the approach and go missed (climb and head for DPK)?  Upon crossing the MAP, is it sufficient to have the ground and/or first set of lead-in lights in sight?  Should the pilot see more than that, such as the actual runway?  If weather temporarily (about 3 to 5 seconds) blocks off the view of the ground or lead-in lights can he continue?  Basically, what is the minimum criteria for being able to continue the approach, after the MAP?  Must the runway be in sight at all times, or is knowledge of position (based on lead-in lights, etc.) enough?

Related: FAR 91.175 Takeoff and Landing under IFR

What you need to have in sight is listed in this question, and what happens if you lose sight of the runway is covered in this answer

This approach is different than most others, in that it has a fly visual section of the approach, as noted in the lower portion of the plan view:

When visual reference established, fly visual to airport via lead-in lights to Rwy 13L or 13R.

When flying the approach, the pilot must have visual contact with the ground, and be able to maintain visual flight by reference to the ground. Arriving at DMYHL, the flight visibility must be at least two miles, and there must be at least an 800-foot ceiling. Considering that DMYHL is either 2.6 or 3.6 NM from the landing runway, it's entirely legal to continue flight without the runway in sight. Without ground contact and the required visibility and ceiling, a missed approach must be executed at DMYHL.
After passing DMYHL, any time the pilot loses visual contact with the ground, or is unable to fly the Fly Visual path as charted (if, for instance, they could see the ground near them but not the lead-in lights) they must execute the missed approach at that time. Note that continuous ground contact must exist - the lead-in lights could disappear behind a small cloud for 3-5 seconds if the pilots still had the approach path in sight, but the aircraft would not be permitted to enter a cloud after the MAP.
For reference, the AIM contains the following information about Fly Visual segments (AIM 5-4-5(l), page 5-4-19 or 345):

l. In  isolated  cases,  an  IAP  may  contain  a  published  visual  flight  path.  These  procedures  are
  annotated “Fly Visual to Airport” or “Fly Visual.” A
  dashed arrow
  indicating the visual flight path will be
  included in
  the profile and plan views with an approximate heading and distance to the end of the runway.
1.
  The depicted ground track associated with the
  “Fly Visual to Airport” segment should be flown as
  a  “Dead  Reckoning”  course.  When  executing  the
  “Fly Visual to Airport” segment, the flight visibility
  must not be less than that prescribed in the IAP; the
  pilot must remain clear of clouds and proceed to the
  airport  maintaining  visual  contact  with  the  ground.
  Altitude on the visual flight path is at the discretion
  of the pilot, and it is the responsibility of the pilot to
  visually  acquire  and  avoid  obstacles  in  the  “Fly
  Visual to Airport” segment.
2.
  Missed  approach  obstacle  clearance  is
  assured only if the missed approach is commenced at
  the  published  MAP.  Before  initiating  an  IAP  that
  contains a “Fly Visual to Airport” segment, the pilot
  should have preplanned climb out options based on
  aircraft  performance  and  terrain  features.  Obstacle
  clearance is the responsibility of the pilot when the
  approach is continued beyond the MAP.
NOTE−
  The  FAA  Administrator  retains  the  authority  to  approve
  instrument approach procedures where the pilot may not
  necessarily have one of the visual references specified in
  14 CFR
  §91.175 and related rules. It is not a function of
  procedure
  design to ensure compliance with
  §91.175. The
  annotation  “Fly  Visual  to  Airport”  provides  relief  from
  §91.175 requirements that the pilot have distinctly visible
  and identifiable visual references prior to descent below
  MDA/DA.

Would it be safe to say that in this particular plate, obstacle clearance is guaranteed anytime a missed approach is commence at or even beyond DMYHL, even though usually "Missed approach obstacle clearance is assured only if the missed approach is commenced at the published MAP" as stated in the AIM?

It looks like there is obstacle clearance: the missed approach path goes over the airport, and in a direction where the highest obstacle within 25 NM is between 1700 and 1800'. However, not every obstacle is printed on the chart, and while you're probably safe, "guaranteed obstacle clearance" has a specific meaning that is not necessarily satisfied after the MAP.

Your MAP is going to be DMYHL waypoint at 2.6NM DME/GPS from Canarsie VOR-DME (CRI) on the CRI 041° radial.  It is also defined at the intersection between CRI 041° radial and Kennedy VOR-DME (JFK) 295° radial.
Also see the upper right corner of the plate:
“MISSED APPROACH: At or beyond MAP, climbing right turn to 4000 and heading 100° to DPK VOR/DME and hold.”
I suspect the unusual configuarion is due to traffic congestion for approaches and departures on those parallel runways to expidite the flow of traffic trying to fly a non precision approach - but don't quote me on that!


text:
Has anyone researched or built an aircraft that controls pitch and roll by pumping liquid within the airframe?
I am confident I have seen an article couple years ago about a "static surface aircraft" (or at least a concept of it) with no control surfaces, which uses a "wet mass" to adjust its roll and pitch.
The container within its CG is filled with a fluid which is then transferred (or sucked) to the wing tips (to control roll) and along the fuselage (to control pitch). I'm not sure about the yaw control - it should be elevon-like.

Having spent half a day online, I have been able to only find a DEMON UAV from BAE systems, a "flapless" aircraft which uses flow injection above the control surfaces for maneuvering. That is not the same.
My question - is there something fundamentally wrong with a concept I have described?
Were there any experimental aircraft made?

The Demon UAV does not use wet mass, it uses compressed air in the place of controls. It's not even close to this concept.

I found this link, about a person who's investigating it for a UAV, but it doesn't look like they're much further than fancy renderings

And I know there are aircraft that have a fuel tank in the tail plane to trim the aircraft, like the Concorde But that is for slow, small, long term balancing control forces, not for rapid, I-need-to-avoid-those-trees-control forces

I really wonder what problem they would be trying to solve.

Weight shift hang gliders come to mind, but that's probably not what you're looking for.

I can't see this being anything other than academic, for example in a 90 degree roll, moving fluid from one wing to the other probably won't cause a correction because now you are just changing the vertical center of gravity.

So far, only pitch. Fuel is pumped between tanks to trim the aircraft. This is especially helpful for supersonic aircraft, because their center of pressure moves by a quarter of the wing chord between sub- and supersonic flight. The prime example would be the Concorde:

… during flight fuel is transfered from tank to tank to maintain trim
  and balance of the aircraft as it does not have a full tailplane which
  would be used on a subsonic airliner to perform this task. Also for
  supersonic flight the Center of Gravity is critial and required to be
  moved for different speeds.(source)
Before take off and during the acceleration through Mach1 to an
  eventual Mach 2, fuel is pumped out of the forward trim tanks to the
  rear trim tanks and the collector tanks in the wings. Around 20 tons
  of fuel is moved in the process and results in a rearward shift of the
  CoG by 6ft (2 meters.)

Concorde fuel tanks and transfer scheme (picture source)
Airbus is using a similar scheme for finely adjusting the center of gravity for optimum performance.
I expect that especially for roll, fluid pumping will not give sufficiently fast response. When flying through gusty air, it is very helpful to have responsive roll control.
If you accept a solid mass rather than a fluid, then the gliders of Otto Lilienthal qualify as the first aircraft which used weight shifting for control, both in pitch and roll. Following his footsteps, every hang glider and trike is using this technique until today.

Otto Lilienthal in his Normalflugapparat, the first serially produced airplane (picture source). Here he throws his legs back in order to pitch up.
Another first used weight shifting, albeit only for pitch control: The very first Zeppelin had a 100 kg weight suspended between its fore and aft gondola which could be shifted fore and aft to adjust the center of gravity.
From www.airships.net:

Pitch was controlled by a sliding weight suspended under the hull
  which could be shifted fore and aft; there were no elevators for pitch
  control, or fins for stability.

The weight proved insufficient, jamming on occasion of the first flight. Later is was increased to 150 kg. Elevators and fins would had helped less because the two 14 hp gasoline engines never allowed the ship to fly fast enough for effective aerodynamic control.

The aerodynamic forces resulting from surface deflections are orders of magnitude larger and faster (in their rate of change) than the gravitational forces obtainable by shifting the centre of mass.
Additionally, as OP seems to realize, yaw control would not be feasible with this method.
So overall it would not be impossible, but it has no clear advantages to outweight the heavy disadvantages. I am also unaware of any prototypes or tech demonstrators using this method.

Nasa's Space shuttle (among many other missile systems) uses pumped liquid to control Pitch, Roll and Elevator
Not in the way that you diagrammed it, though. Instead of using hydraulic control surfaces, it uses thrust vectoring to control all three angles. By sending the Fuel out at a slight difference in angle, or by using RCS to change angles.
Thrust vectoring works by changing the angle that fuel is dejected from the nozzle of the rocket, an thus changing the direction of the thrust. This can change the pitch and yaw of an aircraft (for a rocket, pitch and roll). Controlling roll is another aspect that requires two or more thrusters, which apply a torque to the structure, similar to the way one spins a top.

The question specifies moving fuel "within the aitframe." Thrust vectoring isn't what's being asked about.

@RalphJ rockets still do exactly what op is talking of today, to cancel out torque within the frame while thrusting. Though it's done in space, not in the air.


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Why does a turbofan engine force bypass air into a smaller space?
I asked a question about this same concept recently but this question involves the function of the bypass air.
In this YouTube video around 1 minute in the narrator says that the bypass air is pushed by the fan into a smaller and smaller space which increases the velocity of it, but why? Why increase the exit velocity of the air beyond what the fan already does? It can't increase thrust, can it? That would seem to violate some sort of conservation law since you could theorize an infinitely narrowing space and thrust would approach infinity. So what function does it actually serve to force air into an 'ever narrowing space'?

Related: When is jet engine thrust maximum?

The fan adds energy to the flow in the form of a pressure increase. It is a compressor stage, after all. That pressure is then converted into speed in the nozzle so the flow will (ideally) leave the engine at ambient pressure. The function of the narrowing space is to convert potential energy (pressure) into kinetic energy (speed) which is the basis of thrust creation.

Well if you ask why not let the fan accelerate the flow directly, the answer is, the fan doesn't work very well in a fast flow, but very well in a high pressure flow. So when the flow goes through the fans, it by design does not accelerate much, but increases in pressure a lot. Then when the flow reaches the nozzle, the nozzle converts pressure to velocity. Remember that the flow at inlet is already not far from sonic speed so you really don't want it to go any faster or the fan tips goes supersonic very easily.

@user3528438 So a propeller on a prop plane is much better at increasing the speed of the air going through right? So a follow up question to this is, which would give more thrust at low speeds: a simple propeller or a ducted fan with a nozzle smaller than the output? Considering that they are the same size and run at their optimal rpm.

no, that's why propeller airplanes don't fly very fast. 2) if you only constraint size,or diameter, then ducted fan is way more capable. if you wanted to, you can generate such high pressure that you can jet the flow into supersonic speed, much like a rocket engine but operating with cold compressed air rather than hot combustion gas, even though it will make the device very heavy and not very efficient, but if you only care about size or diameter then that's the answer.


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Is jet fuel typically injected before or after the flame holder in an afterburner?
I don't have the best grasp on the intricacies of afterburners but I'd assume that if the engine has a flame holder then fuel would be injected before it in order to allow it to heat up to autoignition by the time it reaches the flame holder. If not please explain, thanks.

This is what I could find on the matter in my university textbook.

The fuel spray bar is typically composed of
  one or more rings with distinct fuel injection heads circumferentially distributed around
  the ring. The V-shaped flame holder ring(s) create a fuel–air mixture recirculation region
  in its turbulent wake, which allows for a stable flame front to be established.

Although it does not mention fuel heating explicitly, I would agree with your assumption -- as the mixing of the hot gas and the (colder) fuel of course also evens out temperature differences between the two.
Source: Farokhi (2014), Aircraft Propulsion (p. 199)


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Why don't jets fly in the mesophere?
I've seen explanations for why commercial airliners don't reach these altitudes but so far I haven't run across any explanation of how increased speed might make up for low atmospheric density with experimental aircraft.
What if an aircraft was dropping out of orbit, sprouted winglets and turned on its ramjets? Certainly it could greatly increase its time of descent.
I believe there's a simple answer, perhaps the increased weight of fuel or engines can't be made up by airfoil efficiency, etc. So with all due respect what is it?

Commercial aircraft: Likely answered in Why are many jet aircraft designed to cruise around FL350-370? Which additional points would you like to be clarified? For other aircraft: Are there any aircraft designed to operate between an altitude of 50-80km?

I mean, once you get above the stratosphere, the air pressure is practically space. The mesosphere is kind of a pain for scientists because it's really hard to take long-term readings. Too much drag for a satellite, but not enough air to support any form of balloon or aerodynamic flight. Basically limited to sounding rockets and other extremely brief visits.

increased speed might make up for lower atmospheric density

Correct!  This is why true airspeed increases with altitude, while indicated airspeed, or ram pressure, can be constant.
This little trick allows airliners to reach 600 mph while their airspeed gauge reads only 250 mph.
This means the aircraft, pushing through thinner air, gets better miles per gallon until ...
The "sound barrier".  Supersonic flight increases drag.
Next is the "thermal barrier".  At very high speeds passing through air creates a substantial amount of heat from friction.  This is why meteors burn up when they enter the atmosphere.
The air in the Mesosphere is so thin that an aircraft would have to be going thousands of miles per hour to create sufficient lift.  It's been tried with the X-15 experimental aircraft.

"an aircraft would have to be going thousands of miles per hour to create sufficient lift." and the Karman line is the altitude at which the lift created by velocity is smaller than the centrifugal force, and therefore orbiting takes over flying. This is why this line is recognized as the limit atmosphere/space.

Was the X-15 actually able to generate usable lift in the mesosphere, or were they just a rocket at that point, launching through the mesosphere in an aerodynamic stall?

@DarthPseudonym. Many of the X-planes may have been lost for exactly that reason once the engine cut out and the aircraft started to sink.  This is probably why Virgin Galactic went to great lengths to develop their "feathering" system.

Maybe you meant "It's been tried and failed with the X-15", then?  I was under the impression that they never managed to generate significant lift or even aerodynamic control at those altitudes.  IIRC they had RCS thrusters for attitude.

@DarthPseudonym there is something called a flight envelope.  RCS may not have been needed all the time.  The X-15 flew nearly 200 times.  Certainly not a failure.  But they did have issues with heating from atmospheric friction.

@RobertDiGiovanni I'm wondering why less atmosphere/more speed would generate more heat than more atm/less speed. Does it involve the v term in the momentum formula being squared?

@TomRussell it may involve the behavior of each individual molecule hitting the aircraft surface.  And yes, their kinetic energy would be a v$^2$ function.


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Why the plane can not taxi backward?
This is a news from Indonesian  local news
This is the translations from google translate.

Tambolaka - Garuda Indonesia plane seems broken and pushed by people.
  This is a complete explanation from Garuda Indonesia.
The incident was crowded on social media and commented on by various
  netizens. Considering this, Senior Manager of Public Relations of
  Garuda Indonesia Ikhsan Rosan raised his voice and explained it.
"It just happened today, it's at Tambolaka Airport (Southwest Sumba,
  East Nusa Tenggara-red)," he told detikTravel, Friday (8/2/2018).
Ikhsan Rosan asserted, the CRJ type aircraft is not broken. Instead,
  the plane was a little too far as it landed and passed through the
  apron, the aircraft parking lot.
"The plane should be landing go straight to the left to the apron."
  Well, he passed a little, "he added.
Then why should be pushed by people?
"Because there's no pushback car, that's what drives the plane, so it
  seems that it is driven by technicians and airport officers," Ikhsan
  Rosan replied.
Ikhsan Rosan confirmed, Garuda aircraft flight is fine. Everything is
  running smoothly

My Question is : Is it True that the plane can not taxi backward, so if it pass it's apron, people or a push back car must  push the plane back? Why The plane manufacturer do not give the ability to the plane to move backward?

You got it right... planes have engine (s) pushing only in one direction (forward) and wheels are not powered. This is to save costs, and usually is not a problem.

This is unlikely a very standard procedure, to have so many people pushing the airliner by hands. Probably better title it would be "What is going on here, and why?"

The linked question explains that it should be possible. However if they already shut down, it might have been simpler to get people to push it than to start the engines up again and get somebody to indicate to the pilots where to go as they don't have rear visibility and then still burn quite a bit of fuel in the process.


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How is it possible for supersonic aircraft to push air out of the way?
If the force (pressure) can be transferred/propagated only at the speed of sound, how are supersonic aircraft able to move air out of their way/do work on the air? It seems that they must transfer the force faster than the speed of sound or else they would intersect with the air. Does the speed of sound increase at the shock to allow it?

The idea that force can only be transferred/propagated at the speed of sound is a simplification.  It's a very reasonable simplification, which works in a very wide array of cases, but it's a simplification.
A fundamental concept in gas mechanics is the "mean free path length."  This is how far a molecule can travel on average before it collides with another.  This is on the order of hundreds of nanometers for pressures you'd see at reasonable altitudes (68 nm at STP).  If you are looking at processes that occur at scales much larger than this, it is very reasonable to model their behaviors as transmitting things at the speed of sound, because these statistical effects lead to a very natural expected speed of propagation.  It can be seen as a result of the central limit theorem applied to a very large number of molecular collisions where the drift velocity (average movement of particles) is relatively smooth.
As you push things, this nice model breaks down.  As you approach the sound barrier, you get pressure gradients that get shorter and shorter.  Near the speed of sound, these gradients start to get into the single-digit-multiples of this mean path length.  At this point, we can no longer assume that we can just model everything with the nice clean statistical models we had earlier.
What actually ends up happening in most situations is that you get a "shock."  Far from the flying body, the air transmits force at the speed of sound with respect to the unmoving air (or slowly moving, if there's a wind).  Right up against the flying body, the air transmits force at the speed of sound with respect to the flying body.  At "reasonable" speeds, there's a smooth shift from one to the other, where the pressure gradients aren't so steep that we can't call them pressure gradients.
At supersonic speeds, that smooths shift doesn't happen.  Instead, we end up with a thin shock region (on the order of 1 μm or thinner) where it's not reasonable to model the air as having a "pressure."  It acts more like a bunch of billiard balls being flung along.  This region, a few mean free path lengths wide, is where the usual rules we're used to, break down enough to resolve the physics.  On each side of the shock, "normal" speed-of-sound behaviors occur.  Inside the shock, it's messy.

It is only the pressure wave that can propagate at the speed of sound. This means that a molecule of "air" that is ahead of a subsonic aircraft can get pushed out of the way without hitting that aircraft. It gets a push from another molecule, which is pushed by chain of molecules until it get to the one which is the one that hit the aircraft. The aircraft only transfers energy to the molecules that it actually hits.
A supersonic aircraft winds up directly knocking all the molecules ahead of it out of its way, in the form of a shock wave. The aircraft knocks them out of the way at whatever speed it is traveling, and the energy transferred to the whole volume of molecules is the source of the drag rise when speeds approach supersonic.

"these molecules move on their own only at the speed of sound" -- this is not correct. At 25 °C the average speed of a nitrogen molecule is about 515 m/s (calculatiuon here), but the speed of sound is only 346 m/s.

@Henning Makholm When I did 515 * cos(45 degrees) I got the curious number 364.

@HenningMakholm There is no such thing as a molecule of air, so this answer is obviously simplified. If Nitrogen could get out of the way of an aircraft at 515m/s then your comment might apply. The mean free path of Nitrogen at sea level is 34nm, so any given molecule does not move at that speed over any significant distance compared to the size of an aircraft. This is like saying that the water molecules in your body are moving at 590m/s as you get hit by a truck going 100km/h (27m/s); it's true but meaningless. In any case I edited the answer.

Supersonic aircraft do not push the air out of the way in front of the aircraft. They only push it sideways out of the way after the aircraft has passed the air (for the air beside the aircraft), or at the very moment the air meets the aircraft (for the air directly in front of the very centre of the aircraft, i.e. directly in front of the nose spike). That’s why a shock cone extends from the nose backwards, at an angle. The faster the aircraft, the smaller the angle.

The formula relating mach to the shock cone angle can be found here, at NASA's website.
It's no more complex than that.


text:
Has fuel boiling ever been an issue for high altitude aircraft?
Are special measures being taken to prevent fuel from boiling inside the fuel systems at high altitudes? If so, when (with what aircraft) did this practice start?
(question is pertaining to air-breathing, hydrocarbon fueled engine powered aircraft)

Yes, e.g. U2

Both planes, equipped with the older -37 engines, landed at Kirkland Air Force Base Albuquerque, New Mexico. After each development flight, a careful accounting is made of fuel consumption. A special fuel, dubbed lighter fluid, was developed by Shell Oil Company specifically for the Angel, and the finished product was shipped to Nevada in tank cars labeled LF 1A. This blend will not boil at the low pressures encountered at altitude, yet will still give adequate air starts. It is so involatile that fire seldom follows a mishap.
A simple 100 gallon-slipper tank has been developed to fit each wing for extremely long flights. These pressurized tanks contain enough fuel to carry the Angel to cruising altitude where they have no significant effect on speed or range. Even after the addition of an external drag chute, three times the normal oxygen supply, improved breaking and an autopilot, the final all-up weight was within ten pounds of the original proposal.

https://www.cia.gov/library/video-center/video-transcripts/u2-developments.html


text:
Hypothetical aircraft drag reduction
What would happen if I mounted a system dumping hydrogen (sound velocity 1310 m/s) at the leading edges of an aircraft flying at i.e. mach 2 (ignore all practicality issues)? What would happen at the hydrogen/air interface? Would the aircraft experience wave drag?

Consider that, without any hydrogen, the surface of the wing is coated in aluminum, a material with a sound velocity of 6,300 m/s. It's not the speed of sound on the surface of the wing that matters. It's the speed of sound in the medium it's passing through.
You are proposing a wing coated in hydrogen, but even if that could be made, it would still have to move the surrounding air out of the way. It is the "moving the air out of the way" that creates the mach effects.

But effectively, the object is moving through hydrogen - the hydrogen itself would experience wave drag, but to the body it would make no difference whether H2 was stationary or slowed down from it's initial velocity by wave drag, it's still moving through hydrogen below mach 1

@FrancisL.: It's the combination of aircraft plus hydrogen coat that needs to move through air at supersonic speed. Friction might be reduced by the "lubrication" effect of the hydrogen, but wave drag will be much the same, unless you expel so much hydrogen that you leave a thick trailing wake which reduces the effective thickness ratio of the aircraft-hydrogen-combination.

Assuming sufficient flow to engulf the entire airfoil and that the free flow residency time is much shorter than the diffusion time (to assume the injected gas leaves downstream before it can mix, this assumption is likely wrong!) I see two issues:
The airfoil's shape is effectively changed, because the inbound flow now sees the airfoil surrounded by a sheath of hydrogen, which pushes the forward stagnation point forward, to say nothing of the rest of the foil.
The local $Re$ number in the boundary layer is changed due to the different kinematic viscosity of hydrogen. I can't run the math right now, but that will undoubtedly affect the location of the transition point, which will be good or bad depending on the airfoil.
If your goal is to reduce the effective Mach number, then it may work if the first assumptions hold (which is doubtful), but at the cost of a slew of other issues.


text:
Relationship between flapping and lagging frequency

In my rotorcraft module at university, we were told that blades leading/lagging is as a result of blade flapping. To conserve angular momentum, an upward flapped blade (CG moves toward rotor hub) will speed up and hence "lead". A downward flapped blade (CG moves outwards) will slow down and hence "lag". I understand this, however if the lead/lag was as a direct consequence of flapping, then shouldn't their frequencies be the same? In the above image (Shown to us in lecture) you can see that the lagging frequency is roughly 1/5 of the flapping frequency. How is this possible? If the blade lag is a direct consequence of flapping surely this is not possible.
Are there more factors that affect lead/lag? Perhaps cyclic pitch control alters the drag of the blades and hence affects the lead/lag? Or could it be to do with the hinge and any associated damping effects.
A comprehensive explanation would be appreciated.

I agree with your assessment. Have you discussed this with your prof/instructor? When performing Rotor Track and Balance, a worn lead/lag damper or dampers will directly affect the 1 per revolution lateral vibration with little to no affect to other rotor rpm orders or suborders. This is most pronounced in helicopter models which utilize hydraulic dampers as opposed to elastomeric or friction type lead/lag dampers. Hope this helps.
Another factor that contributes to Lead/Lag is alternating spanwise distribution of lift as a result of forward flight. As a rotor blade advances, the lift migrates toward the tip. Conversely, as the blade retreats, lift shifts more toward the root of the blade. Still a 1 per revolution effect.
Chordwise blade mass distribution (Chordwise Balance Aka Product Balance) accounts for angle of attack changes with respect to power changes. If one blade climbes or dives a greater amount than the other/s in transition from Ground-Flat-Pitch to Hover, there is a chordwise balance disparity, which, in turn leads to effects in Lead/Lag. Still a 1 per revolution effect.

The smaller lag frequency mentioned is intended to be the natural frequency that the blade will lead/lag when left alone.  In other words, if you "plucked" the blade tip (pulled it aft a bit and released), it would vibrate in the lead/lag direction at that frequency.
In actual operation, oscillatory forces acting on the blade will occur at other frequencies.  If the rotor is flapping at 1/rev then conservation of momentum will apply a 1/rev oscillatory force in the lead/lag direction.
The total net lead/lag motion of the blade will be a function of both of these and aerodynamic forces, other natural frequencies, ....
As an analogy, consider the A2 string on a guitar.  Its natural frequency is 110Hz (if you pluck it and watch it vibrate).  However, when someone plays guitar they pluck the string at other frequencies.
Another analogy you could analyze mathematically would be a spring-mass system with natural frequency F1, but with an external force acting on the mass with different frequency F2.

Possibly it has to do with the fact that “flapping” also incorporates not just dyssimetry of lift but also control inputs.
Perhaps the formulation comes from the fact that it is calculated for forwards flight?
Control inputs to tilt the disc forwards cause the blade to flap up relative to a hover state of equal pitch in 360deg. Rotation.
The command input to the blade to pitch up and create forwards flight begins as the blade passes through 90deg. From the nose due to gyroscopic precession requirements.  As the blade reaches its maximum flap at 180deg from the nose it begins to descend but is then affected by dyssemetry of lift and tries to flap up again as it passes through 270deg. The point at which it then experiences the maximum dyssemetry.
Thus... the blade experiences more than just a 1 Per/rev for upward flapping events.  While its only downward flap event from a neutral hover pitch angle is while it travels from 000 to 90 Deg...  as it experiences loss of lift due to its retreating blade airflow.


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How are gyros used on a turn indicator?

So I understand how precession works, and that gimbals are used to isolate the gyro from outside rotation, however when a yawing motion is applied to the gyro, it'll essentially translate to a rolling (horizontal) motion on the gimbal to turn the indicator.
Since the gyro maintains its plane of rotation, would it be acceptable to say that if I applied force to a gyro, the precession from the gyro would be displayed on the gimbal not the gyro, since it has to maintain its plane of rotation?
To summarize, I'm just wondering how a gyro's precession affects the gimbals or how it affects the instrument if it doesn't affect the gimbals.
Thank you!

This video may be interesting: How Gyroscopic Turn & Slip Indicator Works

I think the answer you are looking for is here. https://en.wikipedia.org/wiki/Turn_and_slip_indicator#Turn_indicator

The picture given is not complete, it is missing several components. Unlike the other two gyros, the turn coordinator is not allowed to remain rigid in space while the gimbal moves freely around it. The plane of rotation is forced to change with changes in the heading, because of this it does not use a full gimbal. It has one axis other than the gyro core. The housing applying force to the gyro causes precession. In other words, the gyro would like to remain fixed in space but the housing applies force as the aircraft changes heading and bank angle. The resulting precession is restricted by springs.
The higher the rate of change in heading the more torque is generated by the gyro attempting to precess. This force can be measured by the change in spring length, indicated as movement on the gauge. By tilting the free axis up from zero pitch, changes in bank angle will also cause precession. With 90 degrees or a vertical axis the instrument would only precess with changes in bank, angles between 0 and 90 provide a varying mix of the two signals.(Angling the axis down will cause bank to react in the opposite direction, usually this is not desired.)

Suggestion -- maybe try to be more clear about the fact that "turn rate indicator"  and "turn coordinator" are generally not used interchangeably.  Change second sentence to "turn rate indicator".  Then when you talk about tilting the free axis up from zero pitch to sense roll as well as yaw, note that this is the main thing that distinguishes a "turn coordinator" from a "turn rate indicator".  Lousy name, turn "coordinator", because it tends to imply that it has something to do with detecting sideslip, when in fact the slip-skid ball is present on both types of instruments...


text:
Use of Type V platform for regular Air Cargo Transportation with MB-1 chain set
As background, military air transportation is regulated by Technical Orders, Field Manuals, etc. In this case, TO 13C7-1-5 is the one that regulates Platform Delivery System, used for cargo airdrop. To this objective, type V platforms are used due to its special characteristics. However, it is never specified if it can be used as well for cargo transportation without airdropping it. Air cargo transportation is usually done with 463L pallets aka P1P.
Is there any regulation that mentions how to use type V platforms for cargo air transportation instead of 463L pallets? Are these type V platforms only allowed to be used for airdrop?
Last question but not the least: Is there any limitation on the use of MB-1 or MB-2 chain sets with type V platforms if they are to be used for air transport, besides the proper limitation on connection of both systems?
This is a very specific and rare question. I guess Air Force members can clarify this end. Thanks.


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GPS transmission from Aircraft
At the risk of being marked as a duplicate question, i am raising my first ever question in aviation.
I have always wondered how/why of aircrafts positioning, ever since the "lost" plane of Malaysian airline. Here are few things that are still unclear to me -

We have fairly precise GPS service available to cars/phones etc. why cant the same be availale to planes? And more importantly, why cant the planes reciprocate with their own coordinates ? Better if it can be shared in real time ? What are the challenges?

I have read some about ADS-B a little and understand that it has its limitations.  owever, when we have so many human lives at stake, why is a better solution looked into/implemented?
The limitations that have been described for ADS-B systmes is pretty serious, given that many of the aircraft today travel through horizon boundaries.

How exactly do "people" get to know of a aircraft and/or its position in order to "potentially" strike it using a missile etc? Is any of ADS-B data available publicly allowing a plane to be tracked by "anyone"? if so, why also, isnt that encrypted, and if not, why not?

My thought process is mostly around "how can someone track a plane without being part of the ATS system/network"?

What are the areas of our planet not covered by navigational satellites ?  There are references to lack of navigational satellite coverage in many areas on earth.  For that reason, I want to understand more on that topic.  For an aeroplane, flying on a given route, transmitting its position to a satellite above, what are the potential challenges?

Why do you think ADS-B needs to be secret/encrypted? What incidents have occurred that could have been prevented? The only recent incident of a downed airliner almost certainly occurred because the groups in question thought it was military - something that could have been ironically avoided by the use of some kind of flight tracking.

This is really three questions, of which the first is a duplicate, the second doesn't make a huge amount of sense, and the last is unclear. If you have multiple questions, please could you put them into separate threads with one clear, non-duplicate question in each?

Possible duplicate of Why don't all commercial aircraft transmit GPS data in real time?

The duplicate question answers almost all this.  The main point most people don't understand is that GPS only receives.  It does not transmit the position.  An aircraft knows where it is just as well as your car, but no-one can track your car from GPS.

@Simon - As i know, the data is provided anonymously, and therefore i am not trackable, however as a collaborative system, everyone benefits from transmitting their position to the central system core.  I understand that while using my car's GPS, i am receiving as well as providing data to the navigational systems. which is how they can track and tell me where the jam is forming as it happens.

@Dan - Well, if the information is publicly available, it can certainly be misused.  Almost all mishaps have a certain element of "third party involvement", unless confirmed otherwise.  If the ADS-B informaiton is encrypted, that perspective would be highly discounted.

@Raghav You're GROSSLY misunderstanding how GPS and Satellite Navigation work. GPS is one way only - no ifs and buts, all you get from GPS is a Latitude / Longitude location and an estimate of accuracy depending on the amount of satellites involved. Everything else relies on 3rd party propriety systems - that's everything from the map you see to route planning, traffic data and so on. For your domestic sat nav, that either relies on radio transmissions to tell you about traffic (Again,one way only) or internet based communications.

@Raghav But misused how? You're making claims and assumptions that aren't really based in reality - to take down an aircraft in cruise you either need to be on board it, or have access to very serious anti aircraft weaponry. Neither of which are going to be heavily dependent on something like ADS-B.

@Dan - you are absolutely right, without those kind of weaponry, nothing is possible.  I am only looking at the tracking perspective, not on the other areas.  Also, I am not trying to make any claims at all. Just thinking about "potential" problems that can come about if such information is in wrong hands.

the data is provided anonymously.  No data is provided.  GPS satellites do not receive data.  They only transmit data.  The GPS in your car, or on an aircraft, can only receive data.  They might send data to another system, but that is not GPS.  You should carefully read the duplicate and linked questions and answers and also read some basic articles on how GPS works.  You cannot track anything with GPS.

Just want to add that there are no areas of earth not covered by navigational satellites.

Raghav, The planes all over the world can transmit to communication satellites (not GPS satellites) their GPS coordinates but the data traffic made by the planes will be too high. There will be more than 10 000 satellite calls at least twice a minute (an aircraft travels 15 km/min, there could be 10 000 planes in the air at each moment). Such a large number of calls loads too much the existent satellite communication networks.


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What causes the phase to change in a VOR?

I get the light house analogy, and how the VOR essentially works, although I'm wondering what actually causes this phase shift? As it hits magnetic north the variable and reference signal are in phase, as it continues it begins to go out of phase. What's causing it to go out of phase? Is it using constant frequency modulation, where both are being constantly modulated, and at a certain point they both hit the same frequency for a second?

The latest decades VOR are Doppler VOR (DVOR) where the bearing signal is produced by a 14 m diameter circular array of fixed Alford loops antennas. An additional central antenna radiates the reference signal.

Thales Doppler VOR 432, source
DVOR is an improvement of the conventional VOR (CVOR) where the bearing signal is encoded by frequency modulation, more reliable than amplitude modulation used in the CVOR. FM is not produced in the station itself, it is a Doppler artifact created by the apparent displacement of the bearing signal source relatively to the receiver.
DVOR principle
The so-called dual sideband DVOR uses one pair of diametrically opposed antennas from the array at a time, switching to the next pair counter-clockwise at the angular speed of 30 revolutions per second (30 Hz, 1,800 rpm).

One antenna radiates an unmodulated signal at $\small f - 9.96$ kHz ($\small f$ being the frequency of the VOR)

The opposite one radiates at $\small f + 9.96$ kHz.

The receiver sees the two constant frequency sources moving along the circle, one approaching, one moving away. Because they move relatively to the receiver, their frequency is altered by a Doppler shift proportional to the relative velocity.

Doppler shift on the DVOR sidebands antennas
For the receiver, the observed signal frequency is increased for the approaching antenna and decreased for the retreating antenna. By chance for engineers, the Doppler shift applies in opposite quantity on the antennas, and the frequency remains mirrored relatively to the carrier frequency $\small f$ whatever the shift.
At the same time the central antenna radiates a carrier at frequency $\small f$, modulated in AM by the reference signal. This AM signal is not subject to the Doppler shift, as the signal source is fixed.
For the receiver the signal from the central antenna and the two signals from the rotating antennas appear as the three components of single HF signal of frequency $\small f$, modulated in amplitude at 30 Hz (by the reference) and a subcarrier at 9.96 kHz, itself modulated in frequency at 30 Hz (by the variable signal). This signal is therefore entirely compatible with one from a CVOR.
The Doppler shift depends only on the receiver relative bearing
While the timing of the reference is imposed by the VOR, the timing of the subcarrier modulation depends entirely on the aircraft position. E.g. the Doppler shift is null when the axis of the antenna pair is aligned with the aircraft direction, and both antennas move only from left to right from a receiver standpoint:

Doppler shift is null when the receiver is aligned with both antennas</sup
Similarly, and when the distance of the receiver is large compared to the array diameter, the maximum shift is observed when the antenna pair is perpendicular to the previous position (in practical the parallax error can be ignored).
Therefore the delay between the reference and the variable signal is representative of the bearing. Periodic signal phases are actually angles between 0 and 360°, so the bearing is determined at any time by subtracting the current phase of the FM variable signal from the current phase of the AM reference using a phase detector.

Phase of a sine signal
Note the receiver works only by comparison of the phases, their actual values is irrelevant, only the difference is meaningful. If that were not the case, the DVOR wouldn't be compatible with a CVOR receiver.
The CVOR also relies on the phase difference. It works by forming a rotating directional cardioid pattern by space modulation. The result of this modulation faces north when the variable signal which electronically steers the pattern has the value 135°. So the reference was aligned on this value and given the value of 135° for north (in the DVOR north is associated with the reference value 0°).
The diameter of the array gives the variable signal phase its correct value
The tangential velocity when moving along the perimeter of a circle is $\small v = \pi d \times rpm$. Applied to our array: $\small v = \pi \times 14 \times 30 \approx 1,320$. The relative tangential velocity of the sideband antennas is between -1,320 m/s and +1,320 m/s. The value of the Doppler shift for a given velocity is: $\small s = v \cdot f / c$. Applied to a frequency of 110 MHz, the Doppler shift is $\small 1.32e3 \times 1.1e8 / 3e11 = 484$, that is between about -484 Hz and +484 Hz. The exact diameter of the array is actually a bit less, in order to obtain the FM swing of the conventional VOR (480 Hz). This way the DVOR signal can be received by a CVOR receiver (there is no specific DVOR receiver).
As explained earlier, this shift is not encoded by the VOR into the variable signal but added by the Doppler effect in space. However the receiver sees the result and interprets it has if it had been generated by the VOR.
When the never modulated FM subcarrier is nevertheless processed by the receiver demodulator, it produces a 30 Hz sine which phase varies according to the Doppler shift which produced it, therefore depending on the correct diameter of the array.
VOR frequency spectrum
The receiver sees:

A signal at frequency $\small f$, AM modulated by the 30 Hz reference

A signal at $\small f - 9.96$ kHz, FM modulated by a 30 Hz sine (1,800 rpm).

A signal at $\small f + 9.96$ kHz, FM modulated by a 30 Hz sine but with the opposite shift.

It just decides those three signals are components of a single one from a single antenna: The first signal is the carrier, and the two other are sidebands.
It composites the whole thing this way:

This is a carrier of frequency $\small f$, modulated in AM by a 30 Hz signal (reference),

This carrier is also AM modulated by a subcarrier at 9.96 kHz.

The subcarrier is itself FM modulated by a 30 Hz signal (variable) with a modulation swing of 480 Hz.

VOR signal spectrum

Nice explanation!  +1


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Do pilots ever use the aircraft's ICAO 24-bit address when communicating with ATC?
Do pilots know the unique ICAO 24-bit hexadecimal code for their aircraft? Do they ever give it directly to ATC? For example, to controllers who make the flight strips with destination, departure, type of aircraft etc. I know for a fact that some flight plans are sent out that include the unique hex code for the aircraft.

Typically pilots do not know the 24-bit aircraft address of their aircraft.
The 24 bit address is intended for data protocol level of communications and is not used in voice communication. Therefor there is no need for pilots to be aware of their ICAO 24-bit aircraft address. They use the aircraft's registration if they need to identify the aircraft they are using for the flight.
The 24-bit address is used by transponders for communication with Mode-S radars, ADS-B transmissions, by TCAS for tracking traffic and coordinating Resolution Advisories and by Data-Link communications.

Thanks for the fast reply! This actually makes sense as the 24bit address (even when converted to the 6 digit code) would be kinda difficult to remember. However it would be nice if they knew it, so you can link the aircrafts registration to the 24bit address.

Some countries use algorithms that uniquely map the aircraft registration to the 24-bit aircraft address (and vice versa), but many countries do not have any mathematical relation between the registration and the address. There are databases online that keep track of the mappings.

@gary At least in the US, the hex codes are public information. If you look up an aircraft's registration, it will show you the hex code.

@Pondlife the USA is one of countries that use an algorithm to map between aircraft registration and 24-bit aircraft address. Most countries have public aircraft registers that contain the 24-bit address.

@Pondlife and DeltaLima Thanks for the useful information. I'll check to see if I can get access to those registers. In a perfect world we'd all use  a standard algorithm. But that's to much to ask eh.

@gary One source that contains many aircraft and their 24-bit addresses is airframes.org

I will agree that pilots generally do not know the ICAO 24 bit address for their aircraft.  However, with the advent of the ICAO format flight plan, operators who want to take advantage of ADS-B provided ATC services (such as where only ADS-B - no radar - is provided), will have to include the hexadecimal equivalent of their aircraft's 24-bit ICAO address.

Do pilots know the unique ICAO 24-bit hexadecimal code for their aircraft?
Answer depends on the pilot really. Mode S Address(24 bit ICAO address as you say) can be seen as IP of the planes. It is included in some of the Mode S Mode S interrogations/replies as you pointed out.
For commercial airplanes, Mode S address is not changed during lifetime, generally but for some military scenarios, I have read that Mode S Address can be changed per flight for obvious reasons. So you can say, some military pilots definitely know Mode S address of their aircraft.
In addition to that, some platforms may provide you with display and control of Mode S Address.

After reading the question in the title, answer would be no. My answer was for your question in the first sentence of your post.

Well, the transponders I have experience with allowed modifying their Mode S address on the fly. This is the reason i use IP Address analogy. In that sense, Mode S address allocation is not strict as MAC address. I do not have experience with civilian trandponders though. Do they have unchangeable mode s address?

ICAO address on a civil aircraft is assigned as a registration attribute. Transponders can be reprogrammed by maintenance to meet the registration address of the host aircraft when moved to a new airframe. If addresses could be changed in flight this would possibly create a risk of collision due to ACAS perturbation (same for multilateration).


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What jobs are available for a person on the autistic spectrum with ADHD?
For a young man who has Autism and ADHD and considered high functioning ( cognitive and processing difficulties but very social ) what types of " hands on " jobs are there for him in the aviation industry?

Does he like working with his hands and is he good at stuff like building Lego models? (This is not as random as it sounds.)

Related. Are you interested specifically in ATC and/or maintenance (based on your tags)?

This is a very hard to answer question as it very much depends on the person and what you mean by hands on. Can you provide more details? If you want a list of jobs in aviation in general with a brief description of what they involve that might be possible.

It might be worth talking to your local airport to talk to them about options.

It is so tempting to reply "management" but I'll refrain.

pretty much a lot of non-flying jobs come to mind. From cleaning to gate agent to catering to baggage handling. Maybe not the jobs you have in mind. Pilot, controller? Probably not.


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What's the difference between positive and negative pressure relief valves?
In cabin pressurisation systems, I thought that the negative relief valve is automatically opened if outside pressure becomes greater than inside pressure.

There's no such thing as negative pressure!!!!

@Notts90 nope, but there can be negative pressure differential.

@kevin indeed, my pet hate is when people miss off the "differential" and imply there's such a thing as negative pressure :)

The positive pressure relief valve opens in case the cabin pressure is too high in comparison to outside air Pressure. This happens if the pressure differential is higher than 8.85 psi / 0.61 bar (Airbus). It is most likely to happen in malfunction of the outflow valve (stays closed).
The negative pressure relief valve opens most likely during descent, when the cabin altitude is still at 8,000 feet (normal condition) but the aircraft itself is lower than that. That means that the pressure outside of the aircraft is higher than inside. The valve opens at a differential pressure of -1 psi / 0.07 bar.

Aircraft are built to withstand higher pressure on the inside without damage -- think of blowing up a balloon without it popping -- but there is a limit to how much pressure they're designed to take.  Thus the positive pressure relief valve opens if the pressure inside becomes greater than desired, so as to keep if from reaching the point where damage could occur.
On the other hand, the aircraft are NOT designed nearly so robust as far as pressures outside being greater than pressures inside go -- think of the "crushed can" demonstration in science class.  It's rare that this occurs, mainly because digital pressurization controllers are pretty smart, but under the right (wrong!) circumstances, it's not impossible.  Having greater pressure outside the fuselage than in it is considered "negative pressure" and that's a bad thing... thus the negative pressure relief valve opens at a far lower differential than the positive pressure relief valve.

The positive pressure relief-
The positive pressure relief valve opens in case the cabin pressure is too high in comparison to outside air Pressure(pressure differential is higher than 8.85 psi / 0.61 bar). It is most happen in malfunction of the outflow valve (stays closed).
The Negative Pressure relief-
The negative pressure relief valve prevents the generation of sub-atmospheric pressures in the circuit as a result of a discrepancy between the fresh gas flow and the gas evacuation rate (pressure in the cabin from becoming lower than the ambient outside pressure).


text:
Why does the Lockheed D-21B have three air data probes?
The Lockheed D-21 has three air data booms / pitot intake tubes: one on the nose and one on each wing. (Images here and here.)
All three appear to be pitot-static tubes. Anyone know what purpose having three of these served?

most testing/experimental aircraft have wide arrays of extra sensors hanging off them in different positions.

As an autonomous vehicle it needed three probes to enable the FCS to "vote" what the correct signal is if one probe shows erroneous readings.

Peter, since the three probes are in different positions, presumably they would have different readings. A small sideslip, for example, would result in three different yet valid readings, correct?

@ToddStellanova No, parallel probes will read the same value even if in a sideslip. It is easy to see if you make a drawing.


text:
what is the difference between max nose wheel steering angle and effective steering angle?
I am looking at a diagram. The max nose wheel steering angle is 72 deg and effective steering angle is 66.1. My question is now why do we not have just one nose wheel steering angle? what is the difference between max nose wheel steering angle and effective steering angle ? And what angle is used for which cases?

When the two sides of the main landing gear move, each one tends to force a circle with its own radius and center, and is at the same time influenced by the nose landing gear own radius of turn.

Source: A350-900 Aircraft characteristics, airport and maintenance planning
Some tire sliding must occur due to the difference in circles, the actual center is between the three individual centers mentioned and the radius R3 is therefore a bit impacted.
Depending on the differential action on the brakes, and the use of engine thrust asymmetry, the result varies by a few percents (between types of turn 1 and 2 above).


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Difference between multi slats and three slats on B737 ? (leading edge devices)
What is the difference between multi slats and three slats on a B737 ?

Prior to the NG, the total number of slats is 6, that is 3 slats per wing
Thus there are 6 slats numbered from 1 to 6, that is slats 1 and 6 are the outer slat
The NG's have an extra outboard slat on each wing giving 8 in total. They have the same sequencing as the classic
Thus a 3 slats per wing refers to 737 prior to the NG
more precisely
Leading edge devices are comprised of 4 Krueger flaps inboard of the engines and 6 slats outboard of the engines. The LE flaps are extended whenever the TE flaps are not up. The slats extend from 1 to 5 and fully extend when beyond 5. Slat numbers 1 & 6 (the outboard slats) move a few degrees less than slats 2 to 5 when at full extend, causing the leading edge to look slightly disjointed in this configuration, this is normal.


text:
What is the difference between FMCF and AIMS on the 777?
What is the difference between Flight Management Computing Function and Airplane Information Management System on B777?

The B777 uses an Integrated Modular Avionics (IMA) architecture. AIMS is the modular computing platform on the B777. It's basically a bunch of computers in a box with it's own back plane network and operating system. It replaces multiple unique stand alone computing boxes.
FMCF is the software that performs the Flight Management function. It is loaded onto the AIMS platform. In older aircraft architectures (B737, B757, B767) it was contained within a dedicated stand alone Flight Management Computer. In the B787, AIMS was replaced with the CCS (Common Computing System). It also has an FMCF software function.

Thank you very much. It is a lot clearer now.


text:
Meaning of XX below Slat FLap Trim on A350 with flap setting 1
What is the meaning of the 'XX' in amber below the 1 (in green) within the Slat Flap Trim section on the A350 PFD when the flaps are set to 1 with the correct speed (below 255kts)?
See picture attached. If the flaps would have failed, it would indicate XX without the 1. So, what else could this XX indicate?

The number below the flap indication is not the position of the flaps/slats, but the position of the flap lever. When the lever is moved, the number becomes cyan indicating that the flaps/slats are in transit and when they have reached the selected position the number becomes green.
Edit: As per FCOM, there is no mention of amber XX indication for flap lever position at all. Checked in a simulator that total computer failure will not cause indication to turn XX.

This sounds quite reasonable. Do you have some references to support your claim?

I edited my answer. The reference is FCOM DSC-27-20-30. Don't know if there is one available online. Other buses have similar architecture and indications, so you might want to check those from www.smartcockpit.com, for example.

Did you try holding the lever in between detents?

Thats your landing gear indication. The display unit is not receiving data for it. Normally when the gear is up nothing is shown.

You should add references and links to support your answer. Note that this links may provide further reading to dig into the subject of this question.

Why so many downvotes on a post by a new contributer. I can't judge the correctness of the answer, it could very well be right. Yes, references should be included, but that's hardly a reason to downvote. If somebody has the evidence that this answer is faulty,  they should leave a comment with the evidence. @A350Mechanic, welcome to aviation.stackexchange.com


text:
For a small flying wing, can two motors with different speeds replace the ailerons?
I was thinking about a way to get rid of the aileron, and I was wondering if:
I install two motors on the wing, and make the right one spin faster than the left one, will my wing roll to the left ?

Lets replace 2 moving parts with 50. Great idea :)

Related: http://aviation.stackexchange.com/questions/33541/are-fixed-wing-aircraft-with-gimbal-thrust-feasible

@Jamiec - ironically this has long been done in cheap RC toy aircraft, because simple electric motors have fewer moving parts than servo actuators.

Agreed -- to great effect as well.  A good clarification here would be to define how "small" is "small", ranging from 5 lbf (small UAS size), 50 lbf, 100 lbf, to 1500 lbf (GA aircraft size).  With a weight and an assumed distribution thereof, you could actually work out some numbers and compare to another aircraft if needed.  And the answers would vary drastically, I'd think.

First thing first:
I think you mean yaw not roll. See Yaw definition
If you actually mean roll, then the answer is no.
If you mean yaw; then the answer is yes.
However it is impractical for two reasons:
1) Engines, especially jets but also propellers have a latency when speeding up and down. This will make it impractical, and might create pilote induced oscillations.
Electrics motors are more reactive, but so far they have yet to be commonly used in aviation.
2) Redundancy. If one engine fail.... you have to shut down the second engine, and you lost yaw control.

It will not primarily affect roll but it will affect yaw. And if your wing have any yaw-roll coupling (and swept wings certainly do) the yaw will cause your wing to roll a bit but generally not much and generally not completely controllable by you.
Here' one flying wing with no moving control surfaces:

Note that without elevons you're limiting yourself to basically flying level so no snap turns or rolls or even more fancy stunts.


text:
For a flying wing airfoil, which AOA is to consider when selecting Cl?
I'm in the process of designing a flying wing. I'm trying to keep it a small as possible, so the lift coefficient is very crucial for me. I'm currently looking at some airfoils but most of them deliver the good Cl at AOA larger than 0.
My questions:

When deciding a lift coefficient --let's assume 0.5,-- should I choose an airfoil that provides me with a Cl of 0.5 at 0 AOA?
When the flying wing is cruising, the lift force needs to equal the weight, but does the flying wing cruise at 0 AOA or can it cruise at 3° of AOA so it can it can keep its altitude?

The angle of attack also determines the drag amount. In cruise thrust must equal drag (from the whole aircraft, not only the wing). So there is a compromise to be found, possibly having the best L/D ratio in cruise, which determines the angle of incidence. See What is the reason of having an Angle of Incidence on an airplane?, in particular this answer.

Good question. Don't forget to consider the pitching moment (CM) at your design AOA. Otherwise you may have to employ significant elevator control to keep it trimmed.

This book chapter may also help: Wing Design

Do not look at the angle of attack. Instead, focus on ratios like best lift-to-drag ratio. The angle of attack will be a consequence and not a design parameter.
First you need to determine what your flying wing will be good at. Next, determine which propulsion (jet or propeller) it will use. Then you can work on the layout (aspect ratio, wing sweep) and when that is decided, only then you are able to determine the optimum lift coefficient.
The process to determine the optimum lift coefficient is discussed here, here and here.

When deciding a lift coefficient --let's assume 0.5,-- should I
      choose an airfoil that provides me with a Cl of 0.5 at 0 AOA?

No. You should find an airfoil which provided minimum drag at this flying CL. At which AoA the airfoil should operate at a given CL is determined almost completely by camber (2D case)

When the flying wing is cruising, the lift force needs to equal the weight, but does the flying wing cruise at 0 AOA or can it cruise at 3° of AOA so it can keep its altitude?

it can cruise at any pitch attitude given that this attitude does not
   stall the wing.

The answer is not only one: minimum drag at cruise speed, minimum sink, maximum L/D.....as you know, designing a flying machine is the ultimate expression of positive compromise. You are not mentioning at all which planiform the wing will have, its area, wingspan, wing chord (min.& max.), eventual dihedral and/or anhedral, sweep on leading and trailing edges...not enough data given for a decent answer. More information is required before recommending one of more airfoils.


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What factors besides tail/headwinds cause the time taken to fly some routes to be different from the time it takes to return?
I've observed this on many flight trackers; the time taken to fly from A to B is not necessarily the same as the time taken to fly back from B to A. Why is this so and what external factors besides prevailing winds (headwind going one way, tailwind the other) create this difference in time taken?
Example: late May 2015, per American Airlines, PHL to MCO is about 15 minutes longer going south than going north, but times MIA to BDL are essentially identical in either direction.  Prevailing winds would affect both routes similarly, but the difference in times northbound vs southbound isn't the same.  Why not?

Related, if not duplicate: How Earth's rotation affect flight times?

@MadhavSudarshan There are 2 things that haven't been mentioned that can affect flight times. Both are usually of small effect but they can on occasion be of more signfiicant effect. First, for example, a 747-200 at max takeoff weight will only be able to climb to FL290 to FL310 initially. If however, the load is light, you can go right up to, say, FL370 or higher. Also, your sequence in a series of flights bunched up at a popular departure tune along a popular route can make a real difference between the first aircraft and the last insofar as your choice of altitudes.

Sometimes, prevailing winds will account for the difference -- flights going west to east in the US will generally be scheduled for less time than flights going the opposite direction.
The other major factor that can play is in taxi times.  If the delays waiting to get in to a gate matched the times waiting to depart at each airport, these delays would affect both legs equally, but that's not generally the case.  Most of the time, at a congested airport like Atlanta, you can generally get into the gate fairly quickly, but the delays getting airborne can be far longer.
Thus, a flight scheduled from Podunk into ATL might be scheduled for 10 minutes taxi from the gate to departure at Podunk, the flight time, and another 10 or 15 minutes to taxi in, but in the other direction they might build in 30-45 minutes of taxi time departing ATL, plus the flight time, plus 5 minutes landing until blocking in at Podunk.  Thus the leg into ATL is scheduled for flight time plus 25 minutes, while the leg from ATL is scheduled for flight time plus 35-50 minutes.
If it's winter time and your flight was going from BOS or PHL to MCO, another factor in addition to the potential for massive departure delays would be the need to build in time for deicing -- done before departure in PHL or BOS, but not so much leaving Orlando.
Of course, if you didn't need to deice "today," then you'd probably beat the scheduled time by a good bit.  Likewise if the weather was clear and the airport had no delays.  But the mega-hub airports run so close to capacity that any little hiccup in the weather or otherwise causes delays to start building, and then you're using all of your scheduled time, and sometimes more.


text:
What do all the indicators on a HUD mean?
Every Jet Fighter these days seems to have a Head Up Display.  What do all of these indicators mean?
Some things are fairly obvious:

there is a compass
there is an artificial horizon

Other things are less obvious, i.e. what is this _ / \ / \ _ ?

HUD symbology varies depending on flight mode and aircraft configuration so you will find a wide variety of symbols capable of being displayed.  Some basic ones from the F/A-18 E/F aircraft are.

To your original question the ‘W’ symbol is called the Waterline symbol and represents the point at infinity where the longitudinal axis of the aircraft points toward.  It is analogous to the miniature airplane symbol on an attitude indicator.  A similar symbol called the Gun Cross is used on the F-16 HUD.
On high performance aircraft capable of high AoA operations, the waterline symbol is not as useful as the Flight Path Marker or Velocity Vector symbol which indicates the point at infinity the aircraft is currently trending towards.

It indicates where the airplanes longitudinal axis is pointed relative to the horizon and the pitch ladder.

Can’t answer your specific question but I can tell you HUD symbology is often unique to each Service, type aircraft, and manufacturer, so it is possible the specific symbols you are asking about are known only to the pilots, avionics technicians, aircraft software developer, and the small team of military and contractors that acquired and tested the symbology for that aircraft type and model.
Some symbols have evolved to be common across manufacturers but much is unique to fulfill the military customer’s requirement.
So the symbols you ask about could represent vertical or lateral guidance cues, etc. Typical military HUD symbology includes flight parameters such as airspeed, altitude, vertical speed, heading, etc.
Usually critical aircraft system parameters such as engine temperatures, power, etc. are also normally displayed, or at least there are warnings that pop up when display space is limited. Weapons info is also displayed if applicable.
The trade off between space available on the display and what’s deemed required to be in view, without too much clutter, is always in play. This is especially tough when the HUD is helmet mounted as is often the case with military helicopters. That’s a small piece of glass.
Hopefully someone that developed or flew that particular HUD will respond.